1. Field of the Invention
The present invention relates to a data carrier system, and more particularly to a data carrier system which transmits and receives data in accordance with a non-contact manner wherein electromagnetic waves (employing weak radio waves) are used.
2. Description of the Related Art
Heretofore, in physical distribution control such as control of taking out of storage and putting into storage in a plant, or security control for controlling entrance and exit of personnel in facilities such as an office and a plant, it has been proposed to use a data carrier system provided with a tag composed of, for example, an IC card storing prescribed data, a transmitter for transmitting instructions for requesting the data stored by the tag thereto, and a receiver for receiving the data transmitted from the tag on the basis of the requesting instructions derived from the transmitter, whereby the data stored in the tag is effectively used.
As such a data carrier system, for example, the one in which electromagnetic induction is used as the base of the system has been known. A data carrier system wherein electromagnetic induction is applied as the base of the system uses such a principle that when a current is passed through a circular coil-shaped antenna of a controller constituted integrally from the above described transmitter and the receiver, a magnetic field is generated around the coil-shaped antenna, and in such a case if there is a coil on the side of the tag within a range of the magnetic flux of the magnetic field, a current is produced in the coil of the tag side due to changes in the magnetic field appeared around the coil-shaped antenna of the controller.
FIG. 5 shows the constitution of a data carrier system using electromagnetic induction as the basic principle wherein a tag 100 is composed of a memory 102 for storing data consisting of a nonvolatile memory storing prescribed data, a signal control section 104, a modulating-demodulating section 106, an electric power detecting section 108, a first coil 110, and a second coil 112. In this case, the tag 100 does not contain a power source such as a battery and the like.
Furthermore, a plurality of tags 100 are provided for practical use, and they are held by prescribed objects in such a manner that a tag is on board of an article to be transported, or a person carries such a tag.
On the other hand, a controller 200 is composed of a circular coil-shaped antenna 202, a modulating-demodulating section 204, a data processing section 206, a communicating section 208 for communicating with an external computer and the like, and a power source section 210 to which is supplied electric power from an external power source.
First, the controller 200 transmits radio waves for electric power to be fed to the tag 100 from the antenna 202 as the radio wave having a first frequency. As a result, a magnetic field is generated around the antenna 202 as described above, and in this situation, if there is the tag 100 within a range of the magnetic flux in this magnetic field, a current is generated in the second coil 112 of the tag 100 due to changes in the magnetic field appeared around the antenna 202 of the controller 200. In accordance with the process as described above, when the current is generated, the tag 100 takes out electric power from the current through the electric power detecting section 108.
Then, the controller 200 transmits the radio wave having a second frequency different from the first frequency as a control signal for the tag 100. As a consequence, a magnetic field is produced around the antenna 202 as described above, and in this case if there is the tag 100 in a range of the magnetic flux of this magnetic field, a current is produced in the first coil 110 of the tag 100 due to changes in the magnetic field appeared around the antenna 202 of the controller 200. In accordance with the process as described above, when the current representing a control signal is produced in the first coil 110, the tag 100 carries out data processing in the signal control section 104 after demodulating the control signal from the aforesaid current in the modulating-demodulating section 106. The signal control section 104 writes data into or reads data from the memory 102 in response to the contents of the control signal.
In case of writing data, the data is written into the memory 102 in response to the contents of the control signal, while in case of reading data, the data read from the memory 102 is modulated through a modulating circuit involved in the modulating-demodulating section 106, and the data thus modulated is transmitted from the first coil 110 to the controller 200 as radio waves.
Thus, the radio waves transmitted from the first coil 110 are received by the antenna 202 of the controller 200, the radio waves thus received by the antenna 202 are demodulated through the modulating-demodulating section 204, and then they are processed as digital data.
Furthermore, as another data carrier system, the one in which radio waves are used has been also known. FIG. 6 shows the constitution of such a data carrier system employing radio waves in which the radio waves having different two types of frequencies are also used.
The data carrier system shown in FIG. 6 is provided with the number N of tags 300 each having the same constitution as a tag, a receiving apparatus 400, and a transmitting apparatus 500.
The tag 300 is composed of a memory 302 for storing data consisting of a nonvolatile memory in which has been stored prescribed data, a signal control section 304, a modulating-demodulating section 306, a receiving section 308, a transmitting section 310, an antenna 312, and a battery 314.
On the other hand, the receiving apparatus 400 is composed of an antenna 402, a receiving section 404, a demodulating section 406, an information analyzing section 408, a display section 410, and a power source section 412 to which is fed electric power from an external power source. Furthermore, the transmitting apparatus 500 is composed of an antenna 502, a transmitting section 504, a modulating section 506, an information setting section 508, and a power source section 510 to which is fed electric power from an external power source.
As described above, the data carrier system of this type utilizing radio waves uses the radio waves of two different frequencies wherein the radio wave of a first frequency is used as a reminding radio wave requested by the transmitting apparatus 500 for demanding transmission of the data stored in the memory 302 with respect to N of tags 300. More specifically, the reminding radio wave is set by the information setting section 508 to acquire desired data from among the data stored in the memory 302 of the tag 300, the data thus acquired is modulated by the modulating section 506, and then is transmitted from the antenna 502 through the transmitting section 504. The reminding radio wave thus transmitted is received by the receiving section 308, the reminding radio wave received is demodulated by the modulating-demodulating section 306, and then is subjected to data processing in the signal control section 304. The signal control section 304 writes data from the memory 302 in response to the contents of the reminding radio wave.
In the tag 300 which received the reminding radio wave, the data read from the memory 302 is controlled by the signal control section 304 in response to the reminding radio wave received, the controlled data is modulated by the modulating-demodulating section 306, and the data thus modulated is transmitted from the antenna 312 to the receiving apparatus 400 through the transmitting section 310 as the radio wave of a second frequency.
The radio wave of the second frequency to be transmitted to the receiving apparatus 400 is used also for confirming whether or not the tag(s) 300 other than the tag 300 itself which received the reminding radio wave transmit(s) the radio wave of the second frequency within a prescribed period of time in order to avoid simultaneous transmission of the radio wave from the plural tags 300 to the receiving apparatus 400. In the case when the tag(s) 300 other than the tag 300 which received the reminding wave do(es) not transmit the radio wave of the second frequency, the data read from the memory 302 into the tag 300 which received the reminding radio wave as described above is transmitted therefrom as the radio wave of the second frequency.
The radio wave of the second frequency thus transmitted is received by the receiving section 404 through the antenna 402 of the receiving apparatus 400, the data contained in the radio wave of the second frequency is demodulated through the demodulating section 406, the demodulated data is processed as digital data in the information analyzing section 408, and the required contents of the data are displayed on the display section 410.
As described above, the tag 300 of the data carrier system wherein radio waves are used incorporates the battery 314 which usually supplies electric power only to the receiving section 308 of the tag 300 for the sake of achieving low power consumption, and it is adapted to feed the electric power to the whole circuit construction in only the case when the receiving section 308 receives the reminding radio wave.
In the above described data carrier system wherein electromagnetic induction is used as the basic principle, however, an electric current is produced in a coil in the case when both the centers of two coils exist only on the same axes or in the vicinity thereof, so that a positional relationship between the coil of the tag and the antenna (coil) of the controller is inevitably restricted substantially in view of a three dimensional situation. For this reason, there have been such problems that a range for the detection with respect to directions and positions of the tag and the controller is extremely narrowed, and that a distance within which data can be transmitted and received is shortened.
Recently, such a data carrier system having around 1 meter of a reading distance of the data stored in the memory of a tag has also been proposed in data carrier systems wherein electromagnetic induction is used as the basic principle. In the data system proposed, however, a comparatively large amount of electric power is required in case of writing data into the memory of the tag as compared with the case of reading data from the memory, so that the electric power becomes short dependent only upon the electro motive force due to the electromagnetic induction, and as a consequence a distance defined between the coil of the tag and the antenna (coil) of the controller in case of writing data into the memory becomes about 1/5 in case of reading data from the memory.
Furthermore, in recent years, since an increase in the memory capacity of a tag has been intended, an operating time of the tag becomes inevitably longer, hence in this respect also, there has been such a problem that electric power becomes short dependent only upon the electro motive force derived from electromagnetic induction, so that a stable operation of the tag cannot be expected.
In addition to the above, there has been also such a problem that when a plurality of tags exist in a detecting range which can be covered by the antenna of a controller, individual tags cannot be indeed discriminated.
On the other hand, since radio waves are used for transmitting and receiving data in the above described data carrier system wherein radio waves are employed, a distance for transmitting and receiving data can be extended, besides there is no restriction as to positions and directions on the sides of transmission and reception, respectively, and in addition since a battery is used for a tag, there is not a case where the electric power becomes short in the tag. However, the data carrier system of this type has involved such a problem that since it is required to continue always feeding of electric power to the receiving section of a tag, the life of a battery becomes short, so that operations for exchanging the battery or a replacement of the tag and the like operations must be frequently carried out.
Moreover, it is proposed to intend the improvement of processing by reducing a waiting time of each tag with respect to a reminding radio wave in the case where a plurality of tags exist in a detecting range which can be covered by a receiving apparatus. However, there has been such a problem that since there is a limit for reducing a reminding period of time to be assigned to each tag in the case where a number of tags invade within the detecting range, so that individual tags cannot sufficiently be discriminated.